1. Field of the Invention
The invention relates to the field of digital data encoding and decoding.
2. Prior Art
Some digital data encoding techniques use three signal levels to encode two digital states. For instance, Biphase encoding uses a zero (intermediate) signal level to represent a binary 0 and a combination of both positive (high) and negative (low) signals to represent a binary 1. This is achieved in the absence of any DC component independent of the data. However, since binary 0's are represented by a constant intermediate signal, no timing information is transmitted during periods where the data consists of consecutive 0's.
In contrast, the Return-To-Zero Alternate Mark Inversion (RZ-AMI) encoding is the bipolar format used in the "T1" standard and implements a slightly different scheme. Similar to Biphase encoding, RZ-AMI uses a zero signal level to represent a binary 0. However, a binary 1 is represented by a method of alternating high and low signals, which will later be shown, and alternating polarities. Here again, there are no DC components independent of the data and timing information is not transmitted during periods of consecutive 0's.
Another three level encoding scheme is the Non-Return-To-Zero Alternate Mark Inversion (NRZ-AMI). As in Biphase and RZ-AMI, NRZ-AMI uses a zero signal level to represent a binary 0. A binary 1 is represented by a signal alternating between remaining high or low for an entire bit interval. Similar to RZ-AMI encoding, upon the occurrence of a binary 1, NRZ-AMI encoding is alternated to achieve the elimination of DC components independent of the data. Nevertheless, NRZ-AMI encoding also fails to transmit timing information during periods of consecutive 0's.
One common method of decoding digital signals is based on determining whether the amplitude of the signal is above or below predetermined cut-levels. Cut-levels are simply threshold voltages determined by the system designer. In a three-level system (e.g. Biphase), two cut-levels are necessary. For example, when decoding a Biphase signal, if the signal is above the upper predetermined cut-level, or below the lower predetermined cut-level, the result is a binary 1. If the signal is between the two cut-levels, the result is a binary 0.
The problem with this method becomes apparent when long strings of binary 0's are encoded. During this time, the signal remains at approximately a zero level. This results in extended periods of no timing information being transmitted and consequently shifted bit intervals. One way to remedy this problem is commonly known as "stuffing.""Stuffing" requires an entirely different decoding scheme based on the location of, and time equivalent distance (TED) between, the maxima and minima in the signal (rather than the magnitude of the amplitude). Maxima occur when the slope of the signal changes from positive to negative and minima occur when the slope changes from negative to positive. By ascertaining the location of the maxima and minima in the signal, and the TED between them, various "modes" may be engaged as described in the DETAILED DESCRIPTION OF THE PRESENT INVENTION section. The end result is the transmittal of timing information, even during strings of binary 0's.
The properties of these systems as well as others, are discussed in Digital Communication-Second Edition, by Edward A. Lee and David G. Messerschmitt, published by Kluwer Academic Publishers (1994) particularly in Chapter 12 entitled: "Spectrum Control". Additional information pertaining to these systems may be found in Digital Transmission Systems, by David R. Smith, published by Van Norstrand, Reinhold Company (1985) particularly in Chapter 5 entitled: "Baseband Transmission".
Due to distortion and attenuation, digital signals are often altered during transmission. One common undesirable result is baseline wander wherein the decoded signal's zero level begins to drift above or below the actual zero level. One solution to this problem, employed by Biphase encoding, is to utilize a pulse shape having a zero integral.
Another design consideration regarding signal distortion and attenuation is the essential high frequency (f.sub.high). This is the number of cycles per second for a series of all 1's. For Biphase encoding, f.sub.high =bits per second, whereas for NRZ-AMI and R-ZAMI encoding, f.sub.high =(bits per second).div.2 or half the bandwidth required for Biphase encoding.
Other design considerations include whether the chosen encoding method is "self-equalizing"--the tail of the positive-going pulse is effectively canceled by the tail of the negative-going pulse--and the degree to which timing recovery can be simplified. Biphase encoding is "self-equalizing" and since there is a zero crossing in every 1 bit interval, timing recovery is made easy when there are no periods of consecutive 0's. Despite these advantages, and for the same reasons they exist, Biphase encoding requires about twice the bandwidth as NRZ-AMI or RZ-AMI encoding.
As will be seen, the present invention not only provides the advantages of "self-equalization" and simplified timing recovery regardless of the bit pattern, but also is even more effective in reducing baseline wander than Biphase encoding. This is true because the tail of a signal employing the present invention is inherently shorter than the tail of a signal encoded via Biphase. Further, the essential high frequency of the present invention is half of that of Biphase encoding and thus has the above features without requiring a greater transmission bandwidth than NRZ-AMI or RZ-AMI encoding. By employing the "stuffing" technique (discussed above) in conjunction with the present invention, optimal encoding and decoding of digital signals is achieved. Other advantages of the present invention include: lower hardware costs, lower power requirements, and lower data link startup time.